Propane heater burner and pilot go out when thermostat is turned on

I have an older Empire DV-25-5SG wall mount furnace bought second hand earlier this winter. All plumbing outside the unit connected to the tank (I use 30 pound tanks, as they are what I have for my shop) is brand new as of install.
Lately, I have been struggling to keep it going, primary after switching tanks from an empty to a full one. The pilot light lights correctly with the thermostat off, as seen in this photo.
Then, when the thermostat is turned on to set the temp, I can hear the safety magnet click and the burner ignites as it should, as shown in the next photo.
However, after approximately 5 seconds of burning correctly, the burner flame fades and so does the pilot flame, and they begin flickering as shown next:
From here, it flickers until both flames are out, except the pilot light will reappear as soon as the thermostat is turned off and the magnet clicks again.

I have replaced the thermocouple, pilot burner, the gas tubing to the pilot, cleaned/vacuumed out all debris, and cleaned thoroughly the main burner orifice. I've also checked all wiring on the unit and everything seems correct. The unit seems to work when it wants to, sometimes going weeks at a time without working before I try starting it again and it somehow functions until a new tank is put on. At this point, the issues return. I have run out of ideas as to what is wrong, and fear the problem is the gas valve in the unit ($150 replacement I'd rather not make).

Answers 1

We are having the exact problem with our wall mounted propane heater. It ran fine for two years and then we ran out of gas and hooked up a full tank and the pilot lights up but when you turn the knob to fire up the brick for heat the pilot goes out. Then we just turn it off and wait till it decides to work and we'll use it as long as we sre sitting in the room with it.